Preheat oven to 400°F. Soak bread in milk in a small bowl until softened, about four minutes.
Cook up the pancetta (or bacon), onion, and garlic in one tablespoon oil with 1/2 teaspoon each of salt and pepper in a large skillet over medium heat until onion is softened. (This is the step that may lead neighbors to knock and your door and strike up a conversation in the hopes that you will invite them in for a bite of whatever it is you’re cooking).
Cool the mixture slightly.
Squeeze bread to remove excess milk. Lightly beat egg in a large bowl, then combine with chicken, 1 tablespoon tomato paste, pancetta mixture, bread, and parsley. Form 12 meatballs and arrange in another 4-sided sheet pan (Smitten used a 9×13 roasting dish).
Stir together remaining tablespoons of tomato paste and oil and brush over meatballs), then bake in upper third of oven until meatballs are just cooked through, 15 to 20 minutes